Concentrator solar cell having a multi-quantum well system in the depletion region of the cell

ABSTRACT

A solar cell formed from a semiconductor having a relative wide band-gap E b-  characterized by a multi-quantum well system incorporated in the depletion region of the cell in which the quantum wells comprise regions of semiconductor with a smaller band gap separated by small amounts of the wider band-gap semiconductor (E b ) so that the effective band-gap for absorption (E a ) is less than E b .

This invention relates to a concentrator solar cell.

The widespread application of solar cells for electricity generation ismainly limited by the restricted energy conversion efficiency of presentday solar cells and by their expense. It is accepted that the use oflight concentrators can reduce significantly the overall cost of a solarcell system. However a major problem is that the concentration of lightmakes the solar cell much hotter, and the energy conversion efficiencyof a conventional solar cell falls as temperature rises.

According to the present invention a solar cell constructed from asemi-conductor of band-gap E_(b) has a multi-quantum well system formedby the addition, in the depletion region of the cell, of small amountsof a semi-conductor with a smaller band-gap separated by small amountsof the wider band-gap semi-conductor so that the effective band-gap forabsorption (E_(a)) is less that E_(b). The band gaps are chosen so thatat room temperature the quantum efficiency for the collection of chargedcarriers produced by light absorbed in the wells is considerably lessthan 100%. In addition the band-gaps are chosen so that at the higheroperating temperature under concentration this quantum efficiency risesclose to 100%. If appropriately designed the energy conversionefficiency of the present invention will rise with increase oftemperature whereas it falls in a conventional solar cell. By suitablechoice of E_(a) and E_(b) it will be possible to ensure that the chargedcarriers produced by light absorbed in the wells are collected at higherpotential difference than in an equivalent conventional solar cell as aresult of the absorption of thermal energy from interaction with phononsat the operating temperature.

Since these devices are effectively capable of converting heat energy aswell as light energy, they have considerable potential for a much widerrange of applications such as "thermionic generators" and solid-stateself-refrigeration elements or "heat pumps".

A specific embodiment of the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic of the energy band variation across the depletionregion of a p-i-n solar cell of band-gap E_(b) ; and

FIGS. 2(a), 2(b), and 2(c) are diagrams illustrating the dimensions oflayers of various experimental devices.

In FIG. 1, the intrinsic (i) region contains a number (for example,30-100 represented schematically in the figure) of quantum wells formedby small amounts of a lower band-gap semiconductor between small amountsof the solar cell semiconductor of band-gap E_(b). Light with energyE_(ph) greater than the effective band-gap for absorption (E_(a)) isabsorbed in the wells forming electron and hole pairs. In the presenceof the built-in electric field in the depletion region and providing thetemperature is high enough the electrons and holes escape from the wellsand are separated by the electric field to form a current I at a forwardbias V and hence produce useful power IV.

FIGS. 2(a), 2(b), and 2(c) are exemplary devices constructed forexperimental purposes. The aluminium fraction is 33% in all cases forthe AlGaAs, and the quantum wells are formed from GaAs.

Suitable structures could be constructed with combinations or binarysemiconductors and alloys from Groups III and V of the periodic table.In addition, quantum well systems could be made from Group IV alloys(e.g. Si and Ce) and Group II and Group VI alloys (e.g. Cd,Hg andSe,Te).

One possibility is to use Al_(x) Ga_(1-x) As as the barrier material andIn_(y) Ga_(1-y) As as the narrower band-gap material, for example Al₀.3Ga₀.7 As (E_(b) about 1.8 eV), with the narrower band-gap material beingIn₀.15 Ga₀.85 As with E_(a) about 1.2 eV. Under concentrated sunlightthe temperature of such a cell would be expected to rise by about 80° C.Even allowing for the increase in efficiency which results from thehigher light levels under concentration, the theoretical efficiency of aconventional GaAs cell would be expected to fall from 25% to 22% in thissituation. The intrinsic region of the concentrator cell proposed hereshould be of sufficient quality so that the built-in electric field ismaintained into forward bias and so that the non-radiative recombinationlifetime of the carriers generated in the wells is long. If so, thequantum efficiency for collection of carriers absorbed in the wells willincrease from around 30% at room temperature to above 90% at atemperature 80° C. above. On this basis, it is anticipated thatefficiencies higher than 30% should be possible in the inventiondescribed here.

In a typical construction the wells and the intervening barriers areboth 5 to 10 nm wide and the intrinsic region is 500 to 1500 nm wide,these dimensions being measured perpendicular to the planes of thejunctions.

As a further alternative, a combination of InGaAs wells matched to InPbarriers may be expected to provide good results as InP is a goodconventional solar cell material and the "well depths" would be abouttwice those in the AlGaAs/GaAs system.

I claim:
 1. A solar cell formed from a semiconductor having a relativelywide band-gap E_(b), characterized by a multi-quantum well systemincorporated in the depletion region of the cell, in which the quantumwells comprise regions of semiconductor with a smaller band gapseparated by small amounts of the wide band-gap semiconductor (E_(b)) sothat an effective band-gap for absorption (E_(a)) is less than E_(b) ;characterized in that the quantum wells are sufficiently deep to ensurethat electrons and holes trapped in the quantum wells cannot escape atnormal room temperature but are energized to higher levels at highertemperatures so that an energy conversion efficiency of the solar cellis correspondingly increased.
 2. A solar cell according to claim 1comprising a p-i-n device, the quantum wells being incorporated in anintrinsic region of the p-i-n device, so that a built-in electric fieldis maintained in a forward bias region and a recombination lifetime ofcarriers generated in the wells is long.
 3. A solar cell according toclaim 1 wherein the band gaps are chosen so that, at room temperature,the quantum efficiency for collection of charged carriers produced bylight absorbed in the wells is considerably less than 100%.
 4. A solarcell according to claim 1 wherein the wide band gap material is Al_(x)Ga_(1-x) As and the smaller band gap material is In_(y) Ga_(1-y) As. 5.A solar cell according to claim 1 wherein the wide band gap material isInP and the smaller band gap material is InGaAs.
 6. A solar cellaccording to claim 1 wherein the multi-quantum well system includes 30to 100 quantum wells.
 7. A solar cell according to claim 6 in which thewells are 5 to 20 nm wide and the intervening barriers are also 5 to 20nm wide.
 8. A solar cell according to claim 7 in which the intrinsicregion is in the range of 500 to 1500 nm wide.
 9. A semiconductor deviceparticularly adapted for use as a concentrator solar cell or otherenergy conversion applications comprising a semiconductor materialhaving quantum wells of sufficient depth to trap substantially allphoton-generated carriers, whereby the trapped carriers will then absorbphoton energy and escape from the quantum wells to produce a current.10. A solar cell according to claim 9 wherein the semiconductor materialincludes 30 to 100 quantum wells.
 11. A solar cell according to claim 10wherein the quantum wells are 5 to 20 nm wide and the interveningbarriers are also 5 to 20 nm wide.
 12. A solar cell according to claim11 wherein the intrinsic region is in the range of 500 to 1500 nm wide.